When utilizing air springs in passenger cars, a largest possible air volume is to be used to obtain optimal suspension comfort. Mostly, there is insufficient space at the wheel because of chassis components such as a longitudinal control arm, brake and drive shaft. For this reason, this large air volume is subdivided into an air spring volume and an ancillary volume (see FIG. 1a). The ancillary volume can then be accommodated at a location in the vicinity such as in the engine compartment, in the longitudinal support, in the trunk, et cetera. Both volumes are then connected by a line having a cross section which is of such a dimension that an air exchange can take place very rapidly and without significant pressure loss. If the vehicle travels on cobblestones, for example, then the air spring contracts and expands in correspondence to the road speed at a high frequency. Each spring contraction operation and each spring expansion operation is associated with an air exchange which may not be hindered because the suspension comfort would otherwise be reduced.
A high suspension comfort means a reduced spring stiffness. In accordance with the above, this is achieved with a large air spring volume. It is, however, a disadvantage that the steering becomes loose. Likewise, for a low spring stiffness, the driving performance changes when braking, when accelerating, and in travel through a curve as well as with rapid avoidance maneuvers. This change in driving performance is in the direction of instability which is unwanted because driving safety is thereby affected.
In order to resolve this conflict between comfortable air spring design and stability of the driving performance, the above-described line is provided with a valve, which can be blocked (see FIG. 1b). During normal driving conditions, the valve is open and is open in such a manner that the valve presents no significant hindrance for the air exchange between the air spring and the ancillary volume. If the vehicle is now braked, accelerated or driven in a curve or is compelled to execute a rapid defensive maneuver, then the valve is abruptly closed by a control apparatus which can detect the driving state by means of sensors. Thus, the air spring and the ancillary volumes are separated from each other with the consequence that only the air spring volume is available for the suspension operation. The spring stiffness is therefore higher and the vehicle has a more stable driving performance.
The valve is again opened as soon as the control apparatus detects that none of the above-described driving conditions is present any longer. This opening operation has to be carried out in such a manner that a pressure difference between the air spring volume and the ancillary volume, which has possibly formed in the meantime, can be slowly compensated so that there is therefore no sudden drop or upward bucking of the vehicle. Only when the pressure compensation is complete can the valve again be completely opened.
Valves for this task are known. These valves are mostly realized in the assembly of trucks as precontrol valves. A small electromagnetic valve switches a larger pneumatically actuated valve (FIG. 2). The alternative is an electromagnetic actuation of the valve. In the manufacture of passenger cars, no corresponding compressed air source having sufficient power is present in order to switch the pneumatically actuated valve. For this reason, only the electromagnetic actuation remains (FIG. 3).
If one wants to continuously adjust the cross section to be cleared by the valve in order to, for example, obtain specific spring frequencies or to make possible the described slow pressure compensation, then the valve must operate as independently as possible of the existing pressure differences in the system and its inherent friction forces. Forces caused by pressure differences should operate so that they mutually cancel each other and therefore have no influence on the switching or adjusting operation of the valve. Friction forces should be as small as possible and have a constant level. If these requirements are satisfied, then a specific valve setting is assigned to each specific current level supplied to the electromagnet. A continuous clearance of the cross section is thereby provided.
In order to be as independent of pressure as possible, the pressure relief principle shown in FIG. 4 is suitable. A complete pressure relief is, however, not possible (for example, for a star nozzle and a round nozzle), even for the principle illustrated. The reason for this is that a plate membrane would be necessary for pressure relief. The plate membrane, however, has an effective diameter Dw which changes in dependence upon service life (because of stretching) and, in addition, is dependent upon axial and radial built-in tolerances. A roll membrane is not suitable for this purpose because this membrane is turned inside out with a pressure reversal and would thereby be destroyed. This also applies to the plate membrane even though this membrane is somewhat less sensitive.
The same problems (non-constant active diameter and inversion) result also when a slider valve is provided with a plate membrane or roll membrane (FIG. 5). If the slide valve is provided with a seal (FIGS. 6a and 6b), then this seal is burdened with wear and leakage. Likewise, friction forces must be overcome when switching and controlling and these forces change in dependence upon pressure. This can go so far that the friction force is greater than the electromagnetic force and the valve can therefore not switch. A reliably switching valve or a valve wherein each specific current level is assigned to a specific valve position is therefore not realizable therewith.
It is conceivable to utilize a slider valve with a seal (FIGS. 6a and 6b) wherein only small pressure differences occur. In the area of passenger car air spring systems, large pressure differences however occur, which are caused by rapid spring contraction and expansion, so that, at the present time, a use is only possible under reduced requirements.
Furthermore, an electromagnet is required for valve actuation which has a large number of turns with low electrical resistance and therefore has a large valve mass, large structural space and incurs high costs. Also, the armature of the valve would be accelerated toward the valve seat when switching on the actuator current. As a consequence of the large electromagnetic force, which must be made available, large speeds could occur so that large decelerations would become effective when striking the valve seat, that is, the armature generates a noise when striking the valve seat which can be in the nature of a hammer bolt.
In air springs systems for trucks, valves exist for hammer-like closure and slow opening on the basis of pneumatic actuation.
In passenger car air springs, magnetic valves are known which can be adapted to the larger line cross section. Additionally, a pressure relief is provided in order to reduce the acting forces. However, all these solutions are burdened with friction and therefore do not permit a trouble-free adjustment and control. In the manufacture of trucks, the valves are pneumatically actuated because the pneumatic has a high energy density. The high energy consumption (pressurized air escapes) is not significant there. Likewise, the switching noise is also of not much consequence.
It is an object of the invention to provide a motor vehicle air spring wherein the valve used has the following advantageous characteristics, namely:
a) continuous adjustability;
b) independence of existing pressure differences;
c) low leakage;
d) very short reaction time;
e) stable performance in the presence of flow forces;
f) low mass;
g) low friction;
h) adequate service life;
i) small structure;
j) little electrical energy consumed;
k) finely metered continuous opening possible;
l) the complete cross section cleared without throttling;
m) cost effective; and,
n) no disturbing noises.
The motor vehicle air spring of the invention includes: an air spring volume; an ancillary volume; a connecting line connecting the volumes to each other and having a cross section through which air can flow between the volumes; a valve unit mounted in the connecting line; the valve unit having a valve housing defining a valve seat; a valve body movable in the valve housing between a first position wherein the valve body is in contact engagement with the valve seat to close a flow path between the air spring volume and the ancillary volume and a second position wherein the flow path is at least partially open; the valve housing and the valve body conjointly defining an interface region; first and second roll membranes mounted between the valve body and the housing in the interface region; and, the first and second roll membranes being mounted to roll oppositely with respect to each other with the movement of the valve body.
According to the invention, two roll membranes are built into the air spring valve so as to be opposed to each other, whereby the advantageous characteristics and operation described below are obtained.
There is no friction in the adjustment of the valve except for rolling friction and resistance because of rubber deformations. Inversion or destruction of the roll membranes is avoided because of the mutual opposing arrangement thereof. The pressure always operates on the correct side. The space between the two roll membranes communicates with the atmosphere. In this way, it is ensured that the pressure in the rolling lobe is always greater than on the opposite side; that is, on the side between the roll membranes. In this way, an inversion is reliably avoided. Because of the constant effective diameter Dw of the roll membranes, it is possible to design the seat diameter Ds so that all pressure forces always cancel each other. The adjusting force is thereby independent of the actual pressure present in the air spring system. In this way, a trouble-free control is possible without a measurement-technical determination of the pressure. In contrast to plate membranes, the complete pressure compensation is maintained even over the entire service life with the roll membranes according to the invention. The reason for this is that the occurring lengthening does not lead to any change of the effective diameter Dw. Compared to valves having Fowler seals (FIGS. 6a and 6b), the valve of the invention affords the advantage that no leakage can occur. Leakage can occur only via diffusion of the air through the membrane. This leakage is, however, less by many orders of magnitude.
A further advantage is the increased service life. No wear results because of the absence of friction. In this way, the service life is not limited by friction. Because of the non-presence of friction, the valve body can be made of a material having a lower density. As a consequence of the lesser weight, lower acceleration forces (electromagnetic forces) are sufficient so that the electromagnets can be designed smaller. In this way, advantages are, in turn, obtained, namely: smaller structural space for a smaller electromagnet and less switching noise because of a lower mass of the armature and of the valve body. Because of the reduced friction, shorter switching times are possible without simultaneously increasing the electromagnet and the switching noise. Even fewer turns of the electromagnet are sufficient with less current. As a consequence of the fewer turns, the valve is more cost effective, smaller, and lighter than comparable valves. The current can be reduced because of the low friction so that less energy is consumed. The valve is also insensitive to flow forces and has therefore a stable characteristic line.
The use of a step motor as a drive makes possible a precise positioning and an energy switch-off after reaching the desired position. When utilizing a piezo actuator as a drive, the following advantages are obtained: a very low consumption of energy, very high positioning accuracy and very short reaction time. In combination with an electrochemical actuator, the following advantages are obtained: a very low consumption of energy; very high holding forces; high accuracy with respect to positioning even after switching off the energy supply; and, a defined fail-safe condition.
Advantages obtained when implementing with a pneumatic actuator: very short positioning times and a small control valve.
The roll membrane can, preferably, be utilized to completely compensate a star nozzle valve against pressure forces without having to increase the stroke.
The valve of the invention can be used in all other areas where a large cross section has to be completely cleared for short switching times and where only little switching energy is available.